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<h2><a name="SECTION007730000000000000000"></a><a name="immutable-transforms"></a>
<br>
5.7.3 Protocol for automatic conversion to immutable
            
</h2>

<p>
Sets can only contain immutable elements.  For convenience, mutable
<tt class="class">Set</tt> objects are automatically copied to an <tt class="class">ImmutableSet</tt>
before being added as a set element.

<p>
The mechanism is to always add a hashable element, or if it is not
hashable, the element is checked to see if it has an
<tt class="method">__as_immutable__()</tt> method which returns an immutable equivalent.

<p>
Since <tt class="class">Set</tt> objects have a <tt class="method">__as_immutable__()</tt> method
returning an instance of <tt class="class">ImmutableSet</tt>, it is possible to
construct sets of sets.

<p>
A similar mechanism is needed by the <tt class="method">__contains__()</tt> and
<tt class="method">remove()</tt> methods which need to hash an element to check
for membership in a set.  Those methods check an element for hashability
and, if not, check for a <tt class="method">__as_temporarily_immutable__()</tt> method
which returns the element wrapped by a class that provides temporary
methods for <tt class="method">__hash__()</tt>, <tt class="method">__eq__()</tt>, and <tt class="method">__ne__()</tt>.

<p>
The alternate mechanism spares the need to build a separate copy of
the original mutable object.

<p>
<tt class="class">Set</tt> objects implement the <tt class="method">__as_temporarily_immutable__()</tt>
method which returns the <tt class="class">Set</tt> object wrapped by a new class
<tt class="class">_TemporarilyImmutableSet</tt>.

<p>
The two mechanisms for adding hashability are normally invisible to the
user; however, a conflict can arise in a multi-threaded environment
where one thread is updating a set while another has temporarily wrapped it
in <tt class="class">_TemporarilyImmutableSet</tt>.  In other words, sets of mutable sets
are not thread-safe.

<p>

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<span class="release-info">Release 2.5.1, documentation updated on 18th April, 2007.</span>
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